Communication apparatus and communication method

ABSTRACT

A communication apparatus includes a receiving device configured to receive one physical frame in which a plurality of MAC frames are aggregated. This physical frame includes one acknowledgement request frame for the plurality of MAC frames. The apparatus includes an acknowledgement frame forming device configured to form an acknowledgement frame representing reception statuses of the plurality of MAC frames in response to the acknowledgement request frame. The apparatus also includes a transmitting device configured to transmit the acknowledgement frame. This acknowledgement frame includes a compressed acknowledgement frame representing an acknowledgement bitmap having a size equal to a maximum number of MSDUs (MAC Service Data Units) when one MPDU (MAC Protocol Data Unit) corresponds to one MSDU.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.11/200,103, filed Aug. 10, 2005 which claims the benefit of priorityfrom prior Japanese Patent Application No. 2004-234814, filed Aug. 11,2004, the entire contents of each are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication apparatus andcommunication method which perform media access control on the basis ofthe carrier sense information of a physical layer and the carrier senseinformation of a MAC layer.

2. Description of the Related Art

Media access control (MAC) is control for causing a plurality ofcommunication apparatuses which perform communication while sharing thesame medium to decide how to use the medium in transmittingcommunication data. Owing to media access control, even if two or morecommunication apparatuses transmit communication data by using the samemedium at the same time, there is less chance of the occurrence of aphenomenon (collision) in which a communication apparatus on thereceiving side cannot decode communication data. Media access control isalso a technique for controlling access from communication apparatusesto a medium so as to minimize the chance of the occurrence of aphenomenon in which, despite the presence of communication apparatuseshaving transmission requests, the medium is not used by any of thecommunication apparatuses.

In particularly wireless communication, however, media access control(MAC) which is not premised on collision detection is required becauseit is difficult for a communication apparatus to monitor transmissiondata while transmitting the data. IEEE802.11 is a typical technicalstandard for wireless LANs, and uses CSMA/CA (Carrier Sense MultipleAccess with Collision Avoidance). According to CSMA/CA in IEEE802.11, inthe header of a MAC frame, a period (duration) until the end of asequence comprising one or more frame exchanges following the frame isset. In this period, a communication apparatus which is irrelevant tothe sequence and has no transmission right waits for transmission upondetermining a virtual busy state of the medium. This prevents theoccurrence of collision. On the other hand, a communication apparatuswhich has a transmission right in this sequence recognizes that thewireless medium is not used except for a period during which the mediumis actually used. IEEE802.11 defines that the state of a medium isdetermined on the basis of such a combination of virtual carrier senseon a MAC layer and physical carrier sense on a physical layer, and mediaaccess control is performed on the basis of the determination.

IEEE802.11 using CSMA/CA has increased the communication speed mainly bychanging the physical layer protocol. With regard to the 2.4 GHz band,there have been changes from IEEE802.11 (established in 1997, 2 Mbps) toIEEE802.11b (established in 1999, 11 Mbps), and further to IEEE802.11g(established in 2003, 54 Mbps). With regard to the 5 GHZ band, onlyIEEE802.11a (established in 1999, 54 Mbps) exists as a standard. Inorder to develop standard specifications directed to further increasecommunication speeds in both the 2.4 GHz band and the 5 GHz band,IEEE802.11 TGn (Task Group n) has already been established.

In addition, several access control techniques designed to improve QoS(Quality of Service) are known. For example, as a QoS technique ofguaranteeing parameters such as a designated bandwidth and delay time,HCCA (HCF Controlled Channel Access) which is an extended scheme of aconventional polling sequence of IEEE802.11 is available. According toHCCA, scheduling is performed in a polling sequence in consideration ofrequired quality so as to guarantee parameters such as a bandwidth anddelay time. Jpn. Pat. Appln. KOKAI Publication No. 2002-314546 refers toQoS in the IEEE802.11e standard, and discloses a method of assigningpriorities to communications between communication apparatuses in awireless network.

When the same frequency band as that in the existing specifications isto be used in increasing the communication speed, it is preferable toassure coexistence with communication apparatuses conforming to theexisting specifications and to maintain backward compatibility. For thisreason, it is basically preferable that a protocol on a MAC layerconforms to CSMA/CA matching the existing specifications. In this case,a temporal parameter associated with CSMA/CA, e.g., an IFS (InterFrameSpace) or random backoff period needs to match that in the existingspecifications.

Even if an attempt to increase the communication speed in terms ofphysical layer succeeds, the effective throughput of communicationcannot be improved. That is, when the communication speed of thephysical layer is increased, the format of a PHY frame (PHY header andPHY preamble) ceases to be effective any more. An increase in overheaddue to this may hinder an increase in throughput. In a PHY frame, atemporal parameter associated with CSMA/CA is permanently attached to aMAC frame. In addition, a PHY frame header is required for each MACframe.

As a method of reducing overhead and increasing throughput, a Block Acktechnique introduced in recently drafted IEEE802.11e Draft 5.0(enhancement of QoS in IEEE802.11) is available. The Block Ack techniquecan consecutively transmit a plurality of MAC frames without any randombackoff, and hence can reduce the backoff amount to some degree.However, a physical layer header cannot be effectively reduced. Inaddition, according to aggregation introduced in initially draftedIEEE802.11e, both the backoff amount and the physical layer header canbe reduced. However, since the length of a physical layer framecontaining MAC frames cannot be increased beyond about 4 Kbytes underthe conventional limitation on the physical layer, an improvement inefficiency is greatly limited. Even if the length of a PHY layer framecan be increased, another problem arises, i.e., the error tolerancedecreases.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems, and hasas its object to provide a communication apparatus and communicationmethod which can coexist with an existing apparatus and can improve thesubstantial communication throughput by eliminating overheadaccompanying the transmission of a plurality of frames by making a frameformat more efficient.

A communication apparatus according to an aspect of the presentinvention comprises a receiving device configured to receive onephysical frame in which a plurality of MAC frames are aggregated. Thisphysical frame includes one acknowledgement request frame for theplurality of MAC frames. The apparatus includes an acknowledgement frameforming device configured to form an acknowledgement frame representingreception statuses of the plurality of MAC frames in response to theacknowledgement request frame. The apparatus also includes atransmitting device configured to transmit the acknowledgement frame.This acknowledgement frame includes a compressed acknowledgement framerepresenting an acknowledgement bitmap having a size equal to a maximumnumber of MSDUs (MAC Service Data Units) when one MPDU (MAC ProtocolData Unit) corresponds to one MSDU.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram showing the arrangement of a communicationapparatus according to the first embodiment of the present invention;

FIG. 2 is a view showing the frame format of a Block Ack Request definedin IEEE802.11e Draft 8.0;

FIG. 3 is a view showing the frame format of a Block Ack defined inIEEE802.11e Draft 8.0;

FIG. 4 is a view showing an (immediate) Block Ack sequence;

FIG. 5 is a view showing a (delayed) Block Ack sequence;

FIG. 6 is a view for explaining a conventional Block Ack formationprocedure;

FIG. 7 is a view for explaining the conventional Block Ack formationprocedure;

FIG. 8 is a view showing the format of a Compressed Block Ack;

FIG. 9 is a view showing an example of a format which aggregates aplurality of MPDUs;

FIG. 10 is a view showing another example of the format which aggregatesa plurality of MPDUs;

FIG. 11 is a view for explaining a Compressed Block Ack Responseaccording to the first embodiment;

FIG. 12 is a view for explaining the Compressed Block Ack Responseaccording to the first embodiment;

FIG. 13 is a view for explaining the Compressed Block Ack Responseaccording to the first embodiment;

FIG. 14 is a view for explaining retransmission control example 1according to the first embodiment;

FIG. 15 is a view for explaining retransmission control example 1according to the first embodiment;

FIG. 16 is a view for explaining retransmission control example 1according to the first embodiment;

FIG. 17 is a view for explaining retransmission control example 2according to the first embodiment;

FIG. 18 is a view for explaining retransmission control example 2according to the first embodiment;

FIG. 19 is a view for explaining retransmission control example 2according to the first embodiment;

FIG. 20 is a view showing an example of a format which aggregates aplurality of MPDUs according to the second embodiment;

FIG. 21 is a view showing another example of the format which aggregatesa plurality of MPDUs according to the second embodiment;

FIG. 22 is a view for explaining an example of a retransmissionprocedure when a Block Ack Request is omitted according to the secondembodiment;

FIG. 23 is a view for explaining the example of the retransmissionprocedure when a Block Ack Request is omitted according to the secondembodiment;

FIG. 24 is a view for explaining the example of the retransmissionprocedure when a Block Ack Request is omitted according to the secondembodiment;

FIG. 25 is a view for explaining another example of the retransmissionprocedure when a Block Ack Request is omitted according to the secondembodiment;

FIG. 26 is a view for explaining the other example of the retransmissionprocedure when a Block Ack Request is omitted according to the secondembodiment;

FIG. 27 is a view for explaining the other example of the retransmissionprocedure when a Block Ack Request is omitted according to the secondembodiment;

FIG. 28 is a view for explaining the other example of the retransmissionprocedure when a Block Ack Request is omitted according to the secondembodiment; and

FIG. 29 is a view showing buffer management including retransmissionperformed by a data transmitting terminal.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIG. 1 is a block diagram showing the arrangement of a communicationapparatus according to the first embodiment of the present invention. Acommunication apparatus 100 is an apparatus configured to communicatewith another communication apparatus through a wireless link, andincludes processing units 101, 102, and 103 respectively correspondingto a physical layer, MAC layer, and link layer. These processing unitsare implemented as analog or digital electronic circuits in accordancewith implementation requirements. Alternatively, the processing unitsare implemented as firmware or the like to be executed by a CPUincorporated in an LSI. An antenna 104 is connected to the physicallayer processing unit (“processing unit” will be omitted hereinafter)101. The MAC layer 102 includes an aggregation processing device 105 forMAC frames. The aggregation processing device 105 includes a carriersense control device 106 and retransmission control device 107 whichperforms transmission/reception of Block Ack (acknowledgement for aplurality of MAC frames) frames (to be described in detail later) andBlock Ack Request (requirement for acknowledgement) frames (to bedescribed in detail later), retransmission control based on Block Ackand Block Ack Request frames, and the like.

The physical layer 101 is designed to be compatible with two types ofphysical layer protocols. The processing unit 101 includes a first-typephysical layer protocol processing device 109 and a second-type physicallayer protocol processing device 110 for the respective types ofprotocol processing. The first-type physical layer protocol processingdevice 109 and second-type physical layer protocol processing device 110often share circuits and are not necessarily independent of each otherin terms of implementation.

In this embodiment of the present invention, the first-type physicallayer protocol is assumed to be a protocol defined in IEEE802.11a, andthe second-type physical layer protocol is assumed to be a protocolusing a so-called MIMO (Multiple Input Multiple Output) technique usinga plurality of antennas on each of the transmitting side and thereceiving side. Using the MIMO technique makes it possible to expect anincrease in transmission capacity almost proportional to the number ofantennas without changing the frequency band. The MIMO technique istherefore a technique directed to further increase the throughput ofIEEE802.11. Note that the link layer 103 has a normal link layerfunction defined in IEEE802. The technique to be used to increase thetransmission rate is not limited to MIMO. For example, a method ofincreasing the occupied frequency band may be used or may be combinedwith MIMO.

According to IEEE802.11e Draft 8.0, as a technique of improving thetransmission efficiency of the MAC (Media Access Control) layer, a BlockAck (Block acknowledgement) technique has been proposed. In the BlockAck technique, a given terminal transmits QoS (Quality of Service) dataat minimum frame intervals called SIFS (Short InterFrame Space) for agiven channel use period (TXOP: Transmission opportunity). Thereafter,the terminal transmits a Block Ack Request to the receiving terminal atan arbitrary timing to request its reception status. The receiving sideconverts the reception status into information in the bitmap format onthe basis of the Starting Sequence Number determined by the value ofBlock Ack Starting Sequence Control of the Block Ack Request, andreturns the information as a Block Ack.

FIGS. 2 and 3 respectively show the formats of a Block Ack Request frameand Block Ack frame which are defined in IEEE802.11e Draft 8.0. A FrameControl field, Duration/ID field, Receiver Address (destination address)field, and Transmitter Address (transmission source address) field shownin FIGS. 2 and 3 form the MAC header defined in IEEE802.11. A BARControl (Block Ack Request Control) field has a 4-bit TID (TrafficIdentifier: priority identifier) field. QoS data exists for eachpriority (TID) and is assigned a unique sequence number and fragmentnumber. For this reason, a reception status in the Block Ack also needsto be prepared for each priority. The TID field of BAR Control in theBlock Ack Request is used to designate this priority. Block Ack StartingSequence Control (starting point sequence control) in the Block AckRequest in FIG. 2 is comprised of a 4-bit fragment number field and12-bit Starting Sequence Number (start point sequence number) field.Starting sequence number is used by a receiving terminal to generate aBlock Ack by tracing back a reception status, on the basis of a relativereception status from a sequence number corresponding to StartingSequence Number. Like BAR Control in FIG. 2, BA Control in the Block Ackin FIG. 3 contains a 4-bit TID field. Block Ack Starting SequenceControl (starting point sequence control) indicates the starting pointsequence number of the reception status indicated by a Block Ack Bitmap30 in the Block Ack. According to IEEE802.11e Draft 8.0, the size of theBlock Ack Bitmap 30 is a fixed length of 1,024 bits, which makes itpossible to notify a reception status corresponding to data of a maximumof 64 MSDUs (Mac Service Data Units). The Block Ack procedure ofIEEE802.11e takes fragmentation of MAC frame into consideration. Thefragmentation is a process of partitioning a MSDU or MMPDU (MACManagement Protocol Data Unit) into smaller MAC level frames, MPDUs (MACProtocol Data Units). One MSDU can be fragmented into maximum of 16MPDUs. Note that an FCS (Frame Check Sequence) for error detection isadded to each of the MAC frames shown in FIGS. 2 and 3. 1024 is a valueobtained by multiplying the maximum number of MSDUs (“64” inIEEE802.11e) which can be continuously transmitted while using a BlockAck period by the maximum number of fragment frames per MSDU (“16” inIEEE802.11). Therefore, the Block Ack Bitmap 30 has a size of 1,024 bitsin order to notify a reception status of a maximum of 1,024 MPDUs.

FIGS. 4 and 5 each show an example of a Block Ack transmission sequencein HCCA (Hybrid coordination function Controlled Channel Access). An HC(Hybrid Coordinator) shown in each drawing is a QoS access point(QoS-AP) in IEEE802.11e Draft 8.0 and serves as an entity which performsbandwidth management including the allocation of TXOPs to a QoS terminal(QSTA: QoS Station), and performs downlink (the downlink direction fromthe HC to the QSTA) transmission. The assignment of a TXOP to the QSTAis performed on the basis of a QoS CF-Poll frame (QoS ContentionFree-Poll: a QoS-compatible polling frame which is transmitted from theHC to the QSTA to permit transmission). Referring to FIG. 4, first ofall, the HC assigns a channel use period (TXOP 1) to QSTA 1 bytransmitting a QoS CF-Poll frame to it. A QSTA can transmit any frame inits TXOP. In the example shown in FIG. 4, QSTA 1 transmits QoS data atSIFS intervals to QSTA 2. When the TXOP period of QSTA 1 expires, the HCtransmits QoS data to QSTA 1 in a burst manner (TXOP 2). When thechannel period of the HC expires, the HC assigns a channel use period(TXOP 3) to QSTA 1 again. QSTA 1 transmits a Block Ack Request to QSTA2, thereby requesting the destination to transmit a relative receptionstatus designated by Block Ack Starting Sequence Control. FIG. 4 showsan example of Immediate Block Ack. In this case, the terminal which hasreceived the Block Ack Request must return a Block Ack after a SIFSperiod. More specifically, QSTA 2 must return a Block Ack 41 after aSIFS period to a Block Ack Request 40 from QSTA 1. Also, in TXOP 4, QSTA2 must return a Block Ack 43 after a SIFS period to a Block Ack Request42 from the HC.

FIG. 5 shows an example of Delayed Block Ack. Upon receiving a Block AckRequest, the terminal returns a Normal defined in IEEE802.11, andtransmits a Block Ack after a lapse of an arbitrary period. Morespecifically, QSTA 2 having received a Block Ack Request 50 from QSTA 1first returns a Normal 51 defined in IEEE 802.11, and then returns aBlock Ack 52 after a lapse of an arbitrary period. When a datatransmitting terminal having received the last Block Ack returns aNormal, a series of sequences of the Delayed Block Ack are complete.Note that the receiving side is notified of QoS data as an object ofBlock Ack by using an Ack policy field in a QoS control field extendedfor IEEE802.11e Draft 8.0.

A procedure necessary to form a Block Ack will be explained below withreference to FIGS. 6 and 7. Referring to FIG. 6, a transmitting terminaltransmits QoS data in a burst manner, and then transmits a Block AckRequest which designates arbitrary Block Ack Starting Sequence Control(in the example shown in FIG. 6, the sequence number is “1”, and thefragment number is “0”). A receiving terminal in which a receptionstatus is stored for each transmission source address and each priority(TID) tracks back to a frame corresponding to Block Ack StartingSequence Control and, from this frame, forms a relative reception statusas a Block Ack Bitmap having 1,024 bits (for 64 MSDU with fragmentationin mind). The example shown in FIGS. 6 and 7 assumes that thetransmitting side has transmitted an MSDU (fragmented into threeportions) having sequence number “1”, an MSDU (not fragmented) havingsequence number “2”, and the like (total 4 MPDUs). Bit number 60 shownin FIG. 6 indicates a relative position from the start position of aBlock Ack Bitmap. A Block Ack Bitmap 61 in FIG. 6 shows that the MSDU(fragment numbers 63 are “0”, “1”, and “2”) having the sequence number“1” is successfully received, but the MSDU (not fragmented) having thesequence number “2” is not received due to an error or the like. BlockAck Starting Sequence Control 62 of the Block Ack copies the valuedesignated by the Block Ack Request, and transmits the copy. Atransmitting terminal shown in FIG. 7 determines frames to beretransmitted on the basis of Block Ack Starting Sequence Control of aBlock Ack and the contents of a Block Ack Bitmap. In this example shownin FIGS. 6 and 7, the sequence number “2” (not fragmented) is an errorframe, so the corresponding portion of the Block Ack Bitmap 61 is “0”.As a consequence, the transmitting terminal determines that the MSDU(not fragmented) having the sequence number “2” must be retransmitted.

As described above, in the Block Ack determined in IEEE 802.11e Draft8.0, the QoS data receiving side must perform many processes. For thisreason, it is presumably generally difficult to implement ImmediateBlock Ack mechanism by which a Block Ack is returned when an SIFS periodhas elapsed after a Block Ack Request is received. (The receiving sidereceives Block Ack Request frame, performs error check with FCS, checksthe starting sequence number and searches the past reception status fromstorage area, and creates the Block Ack Bitmap for Block Ack frame andtransmits it.)

The embodiment of the present invention, therefore, proposes a method ofsolving this problem. In the first embodiment of the present invention,when a PSDU in which a plurality of MPDUs are aggregated and a Block AckRequest is aggregated at the end of these MPDUs is received, a receptionstatus of the aggregated MPDUs is reflectively returned as an ImmediateBlock Ack.

According to the definition of IEEE802.11, fragmentation (partitioning)is performed if the size of a frame is larger than a predeterminedthreshold value. Fragment numbers are assigned to fragmented MPDUs. Afragment number in the MAC header is a value representing the relativeposition of an MPDU in an MSDU. Fragment numbers normally takeconsecutive values starting from 0. A receiving terminal reassembles theoriginal MSDU on the basis of these fragment numbers and sequencenumbers.

In the MAC transmission procedures of IEEE 802.11 and IEEE 802.11e Draft8.0, fragmented MPDUs are generally transmitted at SIFS intervals, sooverhead corresponding to the frame interval (SIFS) is produced, andthis decreases the transmission efficiency. To realize high throughput,therefore, no fragmentation is desirably performed. Originally, thefragmentation mechanism is installed in an environment that longphysical frame tends to be error-prone. But the aggregation method thatomits IFS and PHY header (and PHY preamble) to reduce the overheadassume that a long physical frame is transmitted normally. In such acase, no fragmentation is desirably performed. Although the fragmentedMPDUs can be aggregated into one physical frame, the overhead of MACheaders still remain. As shown in FIG. 8, in this embodiment whichperforms no fragmentation, the number of bits of a Block Ack Bitmap 80can be compressed to a maximum of 64 MPDUs. That is, the size of theBlock Ack Bitmap 80 is equivalent to the maximum number of MSDUs whenone MPDU corresponds to one MSDU, and can be compressed to 64 bits,i.e., 1/16 the conventional size.

A Block Ack frame as shown in FIG. 8 will be referred to as a“Compressed Block Ack (compressed acknowledgement)” hereinafter. WhenBlock Ack transmission is to be performed between transmitting andreceiving terminals by using a Compressed Block Ack, negotiation mayalso be performed beforehand. As a practical negotiation method, it ispossible to extend, e.g., the Block Ack setting procedure described inIEEE 802.11e Draft 8.0. That is, the use of a Compressed Block Ack isrequested by an ADDBA (add block ack) request, and the use of aCompressed Block Ack is permitted or rejected by an ADDBA response. Whenframes of data, a Block Ack Request, and a Compressed Block Ack asobjects of this setting are to be exchanged, it is possible to imposethe limitation that all these components must respond by CompressedBlock Ack, or to permit them to respond by mixing a normal Block Ack andCompressed Block Ack. It is also possible to add information forrequesting or permitting Compressed Block Ack, rather than normal BlockAck, to a Block Ack Request. The method of requesting or permittingCompressed Block Ack by using a Block Ack Request can be appliedregardless of whether there is a preceding Block Ack setting procedure.

Also, to realize high throughput at the MAC layer, it is possible totransmit individual MPDUs at once by aggregating them into one PSDU.FIGS. 9 and 10 each show an example of aggregation of a plurality ofMPDUs. In the example shown in FIG. 9, the head of each MPDU in a PSDUcontains a field indicating the length of the MPDU and CRC (CyclicRedundancy Check) for the length information of the MPDU. The lengthinformation and CRC of an MPDU will be collectively referred to as an“MPDU separation field” hereinafter. Note that this MPDU separationfield may also contain, e.g., another additional information, apreservation region, and a byte alignment region (for, e.g., 4-bytealignment). A terminal having received this PSDU shown in FIG. 9 checksMPDU separation fields from the first one, and, if an MPDU separationfield is not an error, cuts out the subsequent MPDU, and calculates anFCS (Frame Check Sequence) of the MPDU. If an MPDU separation field isan error, the length of the subsequent MPDU is unknown. Therefore, theterminal continuously calculates (scans) CRCs of MPDU separation fieldsin appropriate byte units. For an MPDU separation field having a correctCRC calculation result, the terminal calculates an FCS of the subsequentMPDU to check whether this MPDU is successfully received.

On the other hand, in the aggregation example shown in FIG. 10, the headof a PSDU contains length information of a plurality of MPDUs as aheader, and CRC is added to a plurality of pieces of length information.This header will be referred to as a “MAC super frame header”hereinafter. A terminal having received this PSDU shown in FIG. 10calculates CRC of the MAC super frame header, and, if the calculationresult is an error, determines that all the MPDUs are errors. If the MACsuper frame header is successfully received, the terminal calculates FCSfor each of the aggregated MPDUs to check whether the MPDU issuccessfully received.

In the examples shown in FIGS. 9 and 10, Block Ack Request frames 90 and101 are aggregated to the ends of the PSDUs in each of which a pluralityof MPDUs are aggregated. Note that in these examples shown in FIGS. 9and 10, the maximum number of MPDUs which can be aggregated is eight.However, this number is of course not limited to eight and can take anyarbitrary value. The maximum number of MPDUs which can be aggregatedmust be predetermined or set by negotiation or the like betweentransmitting and receiving terminals. However, details of a practicalnegotiation method will not be explained.

The basic concept according to the embodiment of the present inventionwill be described below with reference to FIGS. 11 to 13. Assume that atransmitting terminal transmits MPDUs having consecutively assignedsequence numbers “1” to “7”. The transmitting terminal aggregates aplurality of MPDUs (QoS data frames) into one PSDU by the method shownin FIG. 9 or 10, and aggregates a Block Ack Request defined IEEE802.11e111 for these MPDUs to the end of the PSDU. The value of Block AckStarting Sequence Control of the Block Ack Request 111 is the same as asequence number described in the MAC header of the first MPDU aggregatedin the PSDU. The transmitting terminal has a main queue which stores MACframes addressed to various destinations and having various priorities,and a subqueue for retransmitting aggregated MPDUs. In the example shownin FIG. 11, copies of the MPDUs having the sequence numbers “1” to “7”are stored in the subqueue to prepare for retransmission. A terminalhaving received this PSDU in which these MPDUs are aggregated performserror calculations for the MPDUs by the above-mentioned method. In theexample shown in FIG. 11, MPDUs having the sequence numbers “2” and “5”in the PSDU are found to be errors by FCS calculations. In thisembodiment of the present invention, the PSDU receiving terminal formsreception statuses at that point as a Block Ack Bitmap 112. That is, inthe example shown in FIG. 11, a bitmap 1011011 . . . in which “1”indicates successful reception and “0” indicates reception failure isformed. It is of course also possible to switch the positive logic andnegative logic. If the last MPDU aggregated in the PSDU is successfullyreceived, the data receiving terminal forms a Compressed Block Ack 113as shown in FIG. 12 by using a Block Ack Bitmap 112 formed at thatpoint, with respect to the data transmitting terminal.

If the number of the aggregated MPDUs is smaller than the maximum numberof MPDUs which can be aggregated in one PSDU, padding is performed byplacing 0 in the Block Ack Bitmap 112. Assuming that the maximum numberof MPDUs which can be aggregated is 64 in the example shown in FIG. 11,a bit map indicated by 10110110000 . . . is formed. Alternatively inthis embodiment, the receiving terminal can change the bitmap length ofthe Block Ack Bitmap 112 in accordance with the number of MPDUsaggregated in a PSDU. In this case, information indicating the bitmaplength may also be added to the BA Control field of the Compressed BlockAck shown in FIG. 8. The value of the Block Ack Request 111 is copied asthe value of Block Ack Starting Sequence Control of the Compressed BlockAck 113. Referring to FIG. 12, the data transmitting terminal havingreceived the Block Ack compares the value indicated by Block AckStarting Sequence Control of the Compressed Block Ack 113 with thesequence numbers of the MPDUs transmitted by the terminal. From theinformation of the Block Ack Bitmap 112, the data transmitting terminaldetects successfully transmitted MPDUs, and determines MPDUs to beretransmitted. In the example shown in FIG. 12, the Block Ack Bitmap 112contains 0s in portions corresponding to the sequence numbers “2” and“5”. Therefore, the data transmitting terminal determines that these twoframes are MPDUs to be retransmitted.

After thus determining the MPDUs to be retransmitted, the transmittingterminal extracts a new MAC frame 120 from the main queue, assignssequence numbers to the extracted MAC frame 120, and aggregates it in aPSDU, as long as the buffer capacity of the receiving terminal permits.Successfully transmitted MPDUs in the subqueue can be continuouslydeleted from the first one of the queue. The number and the like of thenewly added MPDUs 120 can be determined by using a technique calledsliding window control. A Block Ack Request 122 for a plurality of MPDUsincluding MPDUs 121 to be retransmitted is aggregated to the end of aPSDU in which these MPDUs are aggregated.

FIG. 13 shows a basic sequence example based on the above contents.Within a given TXOP period, a terminal aggregates a plurality of QoSdata into one physical frame, and aggregates a Block Ack Request to theend of a PSDU, thereby urging the receiving side to transmit aCompressed Block Ack immediately. The data receiving side calculatesreception statuses of a plurality of MPDUs except for the Block AckRequest in the PSDU, relates the obtained information directly to aBlock Ack Bitmap, and returns a Compressed Block Ack. More specifically,in TXOP period 1, for example, QSTA 1 aggregates a plurality of QoS datainto one PSDU 130, and aggregates a Block Ack Request 131 to the end ofthe PSDU 130, thereby urging the receiving side to transmit a CompressedBlock Ack immediately. QSTA 2 as the data receiving side calculatesreception statuses of a plurality of MPDUs except for the Block AckRequest 131 in the PSDU 130, relates the obtained information directlyto a Block Ack Bitmap, and returns a Compressed Block Ack 132.

Unlike in the conventional scheme (Block acknowledgement proceduredefined in IEEE802.11e), no past reception status is searched on thebasis of Block Ack Starting Sequence Control of a Block Ack Requestframe. This makes it possible to reduce the search process having arelatively large load, and to facilitate returning a partial responsewithin a SIFS as a limited period. Also, the search time and the circuitscale generally have a tradeoff relationship. Therefore, the circuitscale can be reduced for the same allowable maximum processing delay.

Examples of retransmission control when reception errors occur will beexplained below.

First, retransmission control example 1 will be described with referenceto FIGS. 14 to 16. In this example, assume that a data transmittingterminal aggregates MPDUs having sequence numbers “1” to “7” into onePSDU, aggregates a Block Ack Request to the end of this PSDU, andtransmits it. As shown in the example of FIG. 14, if a Block Ack Request140 is found to be an error by FCS calculation, a receiving terminalcannot return any Compressed Block Ack. In this embodiment of thepresent invention, therefore, a terminal having received a PSDU in whicha plurality of MPDUs are aggregated stores reception statuses of theseMPDUs at that point as Block Ack Bitmap information for one Block Ackresponse, even thought the terminal cannot return any Compressed BlockAck. In the example shown in FIG. 14, the receiving side stores bitmapinformation 141 which is 1011011 . . . for one Compressed Block Ack. Thereceiving side also stores the sequence number of the last MPDU whichthe receiving side has successfully received. In FIG. 14, an MPDU havingthe sequence number “7” is this MPDU. The bitmap information 141 for oneCompressed Block Ack and the sequence number of the last received MPDUare desirably stored for each data transmitting terminal and eachpriority.

If no Compressed Block Ack from the destination can be received evenwhen a predetermined time has elapsed after the PSDU in which aplurality of MPDUs are aggregated is transmitted, the transmittingterminal retransmits a Block Ack Request by using the sequence number ofthe first one of the aggregated MPDUs as the value of Block Ack StartingSequence Control. Normally, it is only necessary to retransmit a BlockAck Request aggregated in an immediately before PSDU. However, if aBlock Ack Request is omitted and hence is not aggregated in animmediately before PSDU as in the second embodiment, it is necessary tonewly form a Block Ack Request. In the example shown in FIG. 14, “1” ofthe first one of the last transmitted MPDUs having the sequence numbers“1” to “7” is described in Block Ack Starting Sequence Control. Also, aBlock Ack Request 142 is basically not aggregated in any other frame. Aterminal having received the Block Ack Request 142 to which no otherMPDU is aggregated compares the value of Block Ack Starting SequenceControl in the Block Ack Request 142 with the value of the sequencenumber of an MPDU which this terminal has successfully received last. Ifthe value of Block Ack Starting Sequence Control is equal to or lessthan (prior to) the sequence number of the last received MPDU, theterminal uses, as Block Ack Bitmap information, the bitmap information141 stored for transmission of one Compressed Block Ack. After a SIFSperiod, as shown in FIG. 15, the terminal reflectively responds by aCompressed Block Ack 150. In the example shown in FIG. 15, the receivingterminal returns the Compressed Block Ack 150 to the transmittingterminal by using, as a Block Ack Bitmap, the reception statuses 1011011of a plurality of MPDUs in the last received PSDU directly. The value ofBlock Ack Starting Sequence Control of the Block Ack Request 142 iscopied as the value of Block Ack Starting Sequence Control of theCompressed Block Ack 150.

In some cases, Block Ack Starting Sequence Control used when the bitmapinformation 141 is formed differs from Block Ack Starting SequenceControl of the Block Ack Request 142. This occurs when the firstaggregated MPDU breaks, the Block Ack Request also breaks, and thebitmap information 141 is formed by assuming that the sequence number ofthe first MPDU which the receiving side has successfully received is thefirst number. In this case, the bitmap information 141 is converted suchthat the sequence number of the start point of the bitmap information141 does not conflict with the Starting Sequence Control of Block AckRequest any longer. Alternatively, the bitmap information 141 is keptunconverted, and Block Ack Starting Sequence Control of the Block Ack tobe returned is set to the value used when the bitmap information 141 isformed (i.e., the value different from the Block Ack Request). Note thatthe receiving side may also store the first sequence number assumed whenthe bitmap information 141 is formed. Even if the first sequence numberis not stored, it can be calculated by an inverse operation by using,e.g., the sequence number of an MPDU which is successfully received lastand the bitmap information 141.

FIG. 16 shows a sequence example according to the above control examplewhen a Block Ack Request is retransmitted. An HC transmits QoS CF-Pollto QSTA 1 to assign it TXOP period 1. Referring to FIG. 16, within therange of the TXOP period, QSTA 1 aggregates a plurality of QoS dataframes and one Block Ack Request into one physical frame 160, andtransmits it to QSTA 2. However, an error occurs in a Block Ack Request161, so no Compressed Block Ack can be received. After that, when TXOPperiod 2 of QSTA 2 expires and the HC assigns TXOP period 3 to QSTA 1again, QSTA 1 retransmits a Block Ack Request 162 and receives aCompressed Block Ack 163 from QSTA 2. In this manner, a series of framesequences are complete.

Retransmission control example 2 will be described below with referenceto FIGS. 17 to 19. Assume, as shown in FIG. 17, that a transmittingterminal transmits MPDUs having sequence numbers “1” to “7” and a BlockAck Request by aggregating them into one PSDU. The receiving sidedetermines as a result of error check that the sequence numbers “2” and“5” and the Block Ack Request are not successfully received, and holds,as bitmap information (in the example shown in FIG. 17, the bit map is1011011), reception statuses of the MPDUs of the PSDU received at thatpoint. The receiving side also stores the sequence number (in theexample shown in FIG. 17, the sequence number “7”) of the last receivedMPDU. The foregoing is the same as in the above-mentioned example.

In the example shown in FIG. 17, the data receiving terminal cannotreturn a Compressed Block Ack. Therefore, if no Compressed Block Ack canbe received even after an elapse of a predetermined time, thetransmitting terminal retransmits a Block Ack Request. As describedabove, the sequence number (in the example shown in FIG. 17, “1”) of thefirst aggregated MPDU is described as the value of Block Ack StartingSequence Control of the Block Ack Request. If no Compressed Block Ack isreturned even though this Block Ack Request is transmitted, thetransmitting terminal gives up retransmission of the data. According toIEEE 802.11e Draft 8.0, QoS data has a Delay Bound corresponding to eachpriority, The Delay Bound specifies the maximum amount of time, in unitsof microseconds, allowed to transport an MSDU, measured between timemarking the arrival of the MSDU at the local MAC sublayer from the localMAC-SAP (Service Access Point) and the time of completion of thesuccessful transmission or retransmission of the MSDU tu thedestination. A MAC frame having exceeded this Delay Bound is discardedby the transmitting terminal, because this MAC frame cannot satisfy QoSrequirement. In the example shown in FIG. 17, if all the MAC frames ofsequence numbers “1” to “7” aggregated and transmitted by thetransmitting terminal have exceeded the Delay Bound and are discarded,new frames are aggregated as the next transmission sequence.

In the example shown in FIG. 18, MPDUs 180 having sequence numbers “8”to “14” and a Block Ack Request 181 are transmitted as they areaggregated into one PSDU. The sequence number “8” of the first MPDU isdescribed as the value of Block Ack Starting Sequence Control of theBlock Ack Request 181 aggregated to the end of the PSDU. If this PSDU inwhich the MPDUs (sequence numbers “8” to “14”) 180 and Block Ack Request181 are aggregated is entirely an error due to collision or the like,the receiving terminal does not update any reception statuses at all. Ifno Compressed Block Ack can be received even after an elapse of apredetermined time, the transmitting terminal retransmits the Block AckRequest 181 in which Block Ack Starting Sequence Control is “8”. Uponreceiving the retransmitted Block Ack Request 181, the receiving sidedetects that the value of Block Ack Starting Sequence Control of theframe 181 is “8” which is larger than the sequence number “7” of thelast MPDU which the receiving side has successfully received. In theexample shown in FIG. 18, bitmap information for one Compressed BlockAck corresponds to the MPDUs having the sequence numbers “1” to “7”, sono previous reception statuses indicated by Block Ack Starting SequenceControl are not recorded at all. In this case, as shown in FIG. 19, thereception statuses (in the example shown in FIG. 18, 1011011) of theMPDUs having the sequence numbers “1” to “7” stored up to the point arecleared by 0, and a Compressed Block Ack 191 in which all bits of aBlock Ack Bitmap 190 are 0 is transmitted. That is, the Block Ack Bitmap190 shows that no MAC frame is successfully received.

Referring to FIG. 19, the terminal which has aggregated and transmittedthe MPDUs having the sequence numbers “8” to “14” receives theCompressed Block Ack 191 from the destination terminal. If all bits ofthe Block Ack Bitmap 190 are 0, all MPDUs 192 having the sequencenumbers “8” to “14” are objects of retransmission. In the example shownin FIG. 19, the MPDUs 192 as objects of retransmission having thesequence numbers “8” to “14” and a Block Ack Request (Block Ack StartingSequence Control is “8”) 193 are aggregated into one PSDU andtransmitted. If the receiving terminal can successfully receive any oneof the MPDUs 192, except for the Block Ack Request 193, aggregated inthe PSDU, it updates the bitmap of the reception statuses and thesequence number of an MPDU which is successfully received last.

In the first embodiment of the present invention described above, it ispossible to reduce bitmap information contained in a Block Ack andincrease the MAC efficiently on the assumption that MSDUs are notfragmented. In this embodiment, the examples of implementation ofImmediate Block Ack in which a Block Ack is returned when a SIFS periodhas elapsed after a Block Ack Request is received have been explained.More specifically, when a PSDU in which a plurality of MPDUs areaggregated and a Block Ack Request is aggregated to the end of theseMPDUs is received, reception statuses can be reflectively returned as anImmediate Block Ack. However, compression of bitmap information withoutany fragmentation is similarly applicable to a Delayed Block Ack.

Also, in this embodiment, one Block Ack Request and one Block Ack arecontained in one PSDU. However, this embodiment may also be extendedsuch that a plurality of Block Ack Requests and a plurality of Block Acks are contained in one PSDU. For example, when a plurality of MPDUsaddressed to the same destination but having different TIDs areaggregated into one PSDU, Block Ack Requests may also be added inone-to-one correspondence with the individual TIDs contained in thePSDU. If some TIDs do not immediately require any Ack or do not requireany Ack at all, the number of TIDs may also be larger than the number ofBlock Ack Request frames. Also, Block Ack frames to be returned are alsogenerated and transmitted in one-to-one correspondence with these BlockAck Requests. Although it is natural to aggregate Block Ack frames to bereturned into one PSDU, they may also be transmitted as different PSDUs.Furthermore, when MPDUs addressed to different destinations areaggregated into one PSDU, Block Ack Requests may also be added inone-to-one correspondence with these destinations contained in the PSDU.Block Acks to be returned are individually transmitted from receivingapparatuses corresponding to the destinations. These Block Ack responsestransmitted from the receiving apparatuses are scheduled so as not tocollide against each other. A plurality of TIDs and a plurality ofdestinations may also be combined.

This embodiment can also be similarly applied to normal CSMA/CA in whichan HC does not control any scheduling.

Second Embodiment

In the first embodiment of the present invention, when a plurality ofMPDUs are aggregated into one PSDU, a Block Ack Request fame is alwaysaggregated to the end of this PSDU. In this case, a terminal havingreceived the PSDU checks errors of the aggregated MPDUs, and, whendetecting the presence of the Block Ack Request, reflectively returns aCompressed Block Ack. By contrast, in the second embodiment of thepresent invention, a plurality of MPDUs are aggregated into one PSDU,and a physical frame is transmitted without aggregating any Block AckRequest frame to the end of the PSDU.

FIGS. 20 and 21 each show an example of the frame arrangement of a PSDUin which a plurality of MPDUs are aggregated. Referring to FIG. 20,information (MPDU length) 201 which indicates the length of an MPDU andCRC 202 for this length information are added to the head of each MPDU.As in the first embodiment, the MPDU length 201 and CRC 202 will becollectively referred to as “MPDU separation” 203 hereinafter. As shownin FIG. 20, in the second embodiment, no Block Ack Request frame existsat the end of a PSDU in which a plurality of MPDUs are aggregated. EachMPDU is regarded as being successfully received if CRC calculation ofthe MPDU separation 203 is correct and the result of calculation of FCSof the MPDU which is indicated by the MPDU length 201 is normal.Referring to FIG. 21, length information of a plurality of MPDUs isadded as a header to the head of the aggregated MPDUs. As in the firstembodiment, this header will be referred to as a “MAC super frameheader” 210 hereinafter. Header CRC 211 is attached to the MAC superframe header 210. If the result of CRC calculation of the MAC superframe header 210 is an error, all the MPDUs are regarded as errors. Notethat the MPDU length information designates the length from the MACheader to FCS in units of bytes. In the second embodiment, as shown inFIGS. 20 and 21, a terminal having received a physical frame in which aplurality of MPDUs are aggregated in a PSDU sandwiched between aphysical (PHY) header (200 in FIG. 20, 212 in FIG. 21) and a physical(PHY) trailer (204 in FIG. 20, 213 in FIG. 21) reflectively returnsreception statuses at that point as a Compressed Block Ack.

The basic transmission/reception sequence of the second embodiment ofthe present invention will be described below with reference to FIGS. 22to 24. In FIG. 22, a transmitting terminal aggregates MPDUs havingconsecutively assigned sequence numbers “1” to “8” into one PSDU, andtransmits this PSDU. As in the first embodiment, the transmittingterminal has a subqueue for retransmission, and stores, in thissubqueue, copies of the MPDUs having the sequence numbers “1” to “8”shown in FIG. 22. In the example shown in FIG. 22, a receiving terminalhaving received the PSDU in which the MPDUs are aggregated calculatesthe reception status of each MPDU, converts the calculated receptionstatuses into a Block Ack Bitmap 220, and reflectively returns aCompressed Block Ack. Unlike in the first embodiment, no Block AckRequest is aggregated. Therefore, if only any one of the MPDUs in thePSDU is successfully received, the receiving terminal returns aCompressed Block Ack when this PSDU. This procedure is referred to as an“Implicit Block Ack Request”. In the example shown in FIG. 22, MPDUshaving the sequence numbers “2”, “5”, and “7” are found to be errors bythe results of FCS calculations. The receiving terminal sets thesequence number of the first MPDU which is successfully received in thePSDU as the value of Block Ack Starting Sequence Control of theCompressed Block Ack. The Block Ack Bitmap 220 is formed from a relativepositional relationship from this MPDU received first. Referring to FIG.22, the bitmap arrangement of the reception statuses is 10110101 . . . .Also, Block Ack Starting Sequence Control of the Compressed Block Ack is“1”. As in the first embodiment described above, if the maximum numberof MPDUs which can be aggregated in one PSDU is not reached, padding orthe like is performed by 0 for the latter half of the bitmap field.Alternatively, the bitmap length of the Block Ack Bitmap 220 may also bevaried in accordance with the number of the MPDUs aggregated in thePSDU. Referring to FIG. 23, the data transmitting terminal havingreceived a Compressed Block Ack 230 from the destination first checksBlock Ack Starting Sequence Control of the frame 230. In the exampleshown in FIG. 23, this value is equal to the sequence number “1” of thefirst MPDU transmitted by the transmitting terminal. Therefore, thetransmitting terminal determines the transmission statuses of the MPDUstransmitted by the terminal on the basis of a Block Ack Bitmap 231 ofthe Compressed Block Ack 230. The Block Ack Bitmap 231 shown in FIG. 23is 10110101. As a consequence, the transmitting terminal detects thatthe MPDUs having the sequence numbers “2”, “5”, and “7” are notcorrectly received. Accordingly, these MPDUs having the sequence numbers“2”, “5”, and “7” are aggregated again as objects of retransmission. Asin the first embodiment, the transmitting terminal also aggregates andtransmits a new frame 232 as long as the buffer capacity of thereceiving side permits. When transmitting a PSDU in which a plurality ofMPDUs and a retransmission frame 233 are aggregated, the transmittingterminal does not aggregate any Block Ack Request to the end of thisPSDU.

As shown in FIG. 24, QSTA 1 assigned TXOP period 1 by an HC transmits toQSTA 2 a physical frame 240 in which a plurality of QoS data areaggregated. QSTA 2 having received the frame 240 returns a CompressedBlock Ack 241 after a lapse of an SIFS period. Downlink transmissionfrom the HC to QSTA 1 is performed following the same procedure. Thatis, in TXOP period 2 of the HC, the HC transmits to QSTA 1 a physicalframe 242 in which a plurality of QoS data are aggregated, and receivesa Compressed Block Ack 243 from QSTA 1, thereby completing a series oftransmission sequences.

In the second embodiment of the present invention, the MAC efficiencycan be increased because no Block Ack Request frame is aggregated in aPSDU. It is also possible to reduce the load of the process of receivinga Block Ack Request frame on the receiving side.

An example of retransmission control when frame errors consecutivelyoccur from the head of a PSDU in which a plurality of MPDUs areaggregated will be explained below with reference to FIGS. 25 and 26. Asshown in FIG. 25, assume that MPDUs having sequence numbers “1” to “8”are aggregated and transmitted, and a receiving terminal determines bythe results of FCS calculations that MPDUs having the sequence numbers“1”, “2”, “5”, and “7” are errors. In the second embodiment, unlike inthe first embodiment, no Block Ack Request frame is aggregated in aPSDU, so the sequence number of the head of the PSDU cannot bedetermined. Especially when an MPDU separation field is added to thehead of a plurality of MPDUs as shown in FIG. 20, the number of MPDUsaggregated in this PSDU is also unknown, so Block Ack Starting SequenceControl cannot be estimated either. In the second embodiment of thepresent invention, therefore, a terminal having received a PSDU in whicha plurality of MPDUs are aggregated sets the sequence number of a firstsuccessfully received MPDU in the PSDU as the value of Block AckStarting Sequence Control of a Compressed Block Ack, and forms a bitmapof reception statuses from a sequence number relation ship relative tothe Block Ack Starting Sequence Control a relative positionalrelationship from this MPDU. That is, as shown in FIG. 25, a first MPDUwhich can be successfully received by a data receiving terminal is aframe having the sequence number “3”. Accordingly, “3” is set as BlockAck Starting Sequence Control of a Compressed Block Ack. In addition, aBlock Ack Bitmap 250 is formed on the basis of a relative positionalrelationship from the MPDU having the sequence number “3” in the PSDU.In the example shown in FIG. 25, a bitmap 110101 is formed. As describedearlier, if the maximum number of MPDUs which can be aggregated in onePSDU is not reached, a rear portion 251 of the bitmap is padded with 0s.Alternatively, the receiving terminal may also vary the bitmap length ofthe Block Ack Bitmap 250 in accordance with the number of MPDUsaggregated in the PSDU. If the transmitting terminal receives aCompressed Block Ack 252 after transmitting a plurality of MPDUs byaggregating them, it compares the sequence numbers of the MPDUstransmitted by it with Block Ack Starting Sequence Control.

In the example shown in FIG. 26, MPDUs having sequence numbers “1” to“8” are transmitted, and the value of Block Ack Starting SequenceControl of the Compressed Block Ack 252 from the destination terminal is“3”. In this case, all MPDUs having sequence numbers less than the valueof Block Ack Starting Sequence Control are regarded as errors. That is,the transmitting terminal determines that MPDUs having the sequencenumbers “1” and “2” are errors. The transmitting terminal alsodetermines that MPDUs having the sequence numbers “5” and “7” areerrors, on the basis of the relative positional relationship of thebitmap starting from the value “3” of Block Ack Starting SequenceControl. Upon receiving the Compressed Block Ack 252 as described above,the transmitting terminal aggregates MPDUs 260 to be retransmitted, and,if the buffer capacity of the receiving side has an extra storage area,aggregates and transmits a new frame.

A case in which a transmitting terminal retransmits a Block Ack Requestwill be explained below with reference to FIG. 27. As described in thefirst embodiment, if no Compressed Block Ack is received from thedestination, a transmitting terminal retransmits a Block Ack Request. Inthe second embodiment of the present invention, when QoS data areaggregated into one physical frame, no Block Ack Request is contained inthe end of this frame. Therefore, a Block Ack Request is retransmittedfrom a data transmitting terminal only when no Compressed Block Ack canbe received. In the example shown in FIG. 27, MPDUs having sequencenumbers “1”, “2”, “5”, and “7” are aggregated and transmitted as aretransmission frame 260. In this case, the value of the sequence numberof a first MPDU of a last transmitted PSDU is copied to Block AckStarting Sequence Control of a Block Ack Request 270. Referring to FIG.27, Block Ack Starting Sequence Control of the Block Ack Request 270 is“1”. In a terminal having received the Block Ack Request 270, thereception statuses of a plurality of MPDUs in a last received PSDU arestored as one Compressed Block Ack response. If the value of Block AckStarting Sequence Control of the Block Ack Request 270 is less than thesequence number of an MPDU which this receiving terminal hassuccessfully received last, the receiving terminal converts the storedbitmap information directly into a Block Ack Bitmap, and reflectivelyreturns a Compressed Block Ack. In the second embodiment, the value ofBlock Ack Starting Sequence Control of this Compressed Block Ack is notcopied from the Block Ack Request. That is, the sequence number of afirst MPDU which is successfully received in a last received PSDU isdescribed as this value. This is so because an MPDU corresponding toBlock Ack Starting Sequence Control indicated by a Block Ack Request isnot always received successfully. Alternatively, it is also possible tocopy Block Ack Starting Sequence Control from the Block Ack Request, andchange the bitmap information to have contents indicating that a portionolder than the first sequence number of the stored bitmap information isnot successfully received.

FIG. 28 shows the basic frame exchange sequence including aretransmission process of the second embodiment of the presentinvention. Referring to FIG. 28, a data transmitting terminal aggregatesMPDUs having sequence numbers “1” to “8” into one PSDU 280, andtransmits it. The receiving side receives the PSDU 280 and, if MPDUshaving the sequence numbers “1”, “2”, “5”, and “7” are errors, transmitsa Compressed Block Ack 281 having a bitmap arrangement 11010100. Thetransmitting side determines from the contents of the Compressed BlockAck 281 that the MPDUs having the sequence numbers “1”, “2”, “5”, and“7” are not successfully received, and retransmits a PSDU 282 byaggregating these MPDUs to be retransmitted. If the receiving sidecannot receive the retransmitted PSDU 282 at all due to collision or thelike, the transmitting terminal transmits, after a predetermined timehas elapsed, a Block Ack Request 283 by setting “1” as Block AckStarting Sequence Control. Although the value of Block Ack StartingSequence Control is “1”, the sequence number of the first one of theMPDUs in the PSDU which the receiving terminal has received last is “3”.Therefore, the terminal having received the Block Ack Request 283 sets“3” as the value of Block Ack Starting Sequence Control of a CompressedBlock Ack 284, and returns a bitmap of stored reception statuses as aBlock Ack Bitmap. The transmitting terminal having received theCompressed Block Ack 284 regards all those MPDUs in the last transmittedMPDUs 282, which have sequence numbers less than the value of Block AckStarting Sequence Control of the Compressed Block Ack 284 as errors, andalso detects error MPDUs from the Block Ack Bitmap. That is, in theexample shown in FIG. 28, all the retransmitted MPDUs 282 are errors. Asa consequence, the transmitting terminal regards all the MPDUs havingthe sequence numbers “1”, “2”, “5”, and “7” as objects ofretransmission, and transmits a PSDU 285 in which these MPDUs areaggregated. After that, the transmitting terminal receives a CompressedBlock Ack 286 from the destination, thereby completing a series of frameexchange sequences.

FIG. 29 shows transmission buffer management including retransmissionperformed by a data transmitting terminal. The data transmittingterminal extracts MAC frames from a main queue including MAC frameshaving various destinations and various priorities, assigns consecutivesequence numbers “1” to “8” to the extracted MAC frames, and stores themin a subqueue 290 for retransmission. The transmitting terminal thenextracts copies of the MPDUs having the sequence numbers “1” to “8”,aggregates these copies into one PSDU, and transmits the PSDU. Afterthat, the transmitting side stores the sequence numbers of thetransmitted MPDUs. In the example shown in FIG. 29, informationindicating “1”, “2”, “3”, “4”, “5”, “6”, “7”, and “8” is stored on thetransmitting side. If a Compressed Block Ack 291 in which Block AckStarting Sequence Control is “3” is received from the destinationterminal, the transmitting terminal regards both MPDUs having thesequence numbers “1” and “2” less than “3” as errors. The transmittingterminal also regards MPDUs having the sequence numbers “5” and “7” aserrors from the Block Ack Bitmap. MPDUs can be deleted backward from theretransmission subqueue 290 if these MPDUs are continuously successfullytransmitted from the first one. In the example shown in FIG. 29, theMPDU having the sequence number “1” is found to be an error, so no framecan be deleted from the subqueue. In this case, successfully transmittedMAC frames in the subqueue 290 are desirably kept stored in the subqueueby giving them some identification information indicating transmissionsuccess. This is so because when a Compressed Block Ack is received inthe next sequence, it becomes difficult to distinguish betweensuccessfully transmitted MPDUs and MPDUs which are not successfullytransmitted. The entity of a successfully transmitted MPDU need notalways be stored. The state of each sequence number is important. Afterthat, the transmitting terminal retransmits the MPDUs having thesequence numbers “1”, “2”, “5”, and “7”. If the receiving side cansuccessfully receive all these MPDUs except for the MPDU having thesequence number “7”, a Compressed Block Ack 292 in which Block AckStarting Sequence Control is “1” is returned. The transmitting terminaldetermines that the MPDUs having the sequence numbers “1”, “2”, and “5”are successfully transmitted, and continuously deletes MPDUs from thefirst one to the one having the sequence number “6” from the subqueue290. Consequently, the MPDU having the sequence number “7” is stored inthe head of the subqueue 290.

In the second embodiment of the present invention as has been explainedabove, the MAC efficiency increases because a Block Ack Request can bedeleted from a PSDU. Also, when an MSDU is not fragmented, it ispossible to reduce bitmap information contained in a Block Ack, andincrease the MAC efficiency. In addition, it is possible to realizeImmediate Block Ack by which Block Ack is returned when an SIFS periodhas elapsed after a Block Ack Request is received. The method ofincreasing the MAC efficiency by deleting a Block Ack Request from aPSDU is also applicable to Delayed Block Ack.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A wireless communication apparatus comprising: a physical frameforming device configured to form one physical frame in which aplurality of MAC frames are aggregated; a transmitting device configuredto transmit the physical frame; and a receiving device configured toreceive an acknowledgement frame which is a frame responding to animplicit acknowledgement request of the physical frame.
 2. The apparatusaccording to claim 1, further comprising an acknowledgement requesttransmitting device configured to, if the acknowledgement frame cannotbe received even when a predetermined time has elapsed after thephysical frame is transmitted, set a sequence number of a first MACframe in the physical frame as a value of acknowledgement start pointsequence control, and transmit an explicit acknowledgement requestframe.
 3. The apparatus according to claim 2, further comprising aretransmitting device configured to retransmit all said plurality of MACframes, if an acknowledgement bitmap contained in an acknowledgementframe responding to the explicit acknowledgement request frame andreceived by the receiving device indicates that no MAC frame issuccessfully received.
 4. The apparatus according to claim 1, furthercomprising a retransmitting device configured to retransmit anunacknowledged MAC frame having a sequence number smaller than a valueof acknowledgement start point sequence control in the acknowledgementframe received by the receiving device.
 5. The apparatus according toclaim 1, further comprising: a subqueue to store a plurality of MACframes to be retransmitted; a sequence number storage device configuredto store sequence number information of the MAC frames aggregated in thephysical frame transmitted by the transmitting device; a determiningdevice configured to determine a MAC frame to be retransmitted, on thebasis of an acknowledgement bitmap in the acknowledgement frame receivedby the receiving device, and the sequence number stored in the sequencenumber storage device; and a deleting device configured to deletecontinuously successfully transmitted MAC frames from a head of thesubqueue, and add a MAC frame to be transmitted next to the subqueue. 6.A wireless communication apparatus comprising: a receiving deviceconfigured to receive one physical frame in which a plurality of MACframes are aggregated; a forming device configured to form anacknowledgement frame which is a frame responding to an implicitacknowledgement request of the physical frame received by the receivingdevice, and contains reception statuses of said plurality of MAC framesin the physical frame as an acknowledgement bitmap, and in which asequence number of a first successfully received MAC frame in thephysical frame is set as an acknowledgement start point sequence number;and a transmitting device configured to transmit the acknowledgementframe.
 7. The apparatus according to claim 6, wherein theacknowledgement frame is a compressed acknowledgement frame representingan acknowledgement bitmap having a size equal to a maximum number of MACService Data Units (MSDUs) when one MAC Protocol Data Unit (MPDU)corresponds to one MSDU.
 8. The apparatus according to claim 6, furthercomprising: a storage device configured to store the acknowledgementbitmap of the acknowledgement frame transmitted by the transmittingdevice, and wherein the receiving device receives an explicitacknowledgement request frame, and the forming device forms anacknowledgement frame representing reception statuses based on theacknowledgement bitmap stored in the storage device, in response to theexplicit acknowledgement request frame.
 9. The apparatus according toclaim 8, wherein if a value of acknowledgement start point sequencecontrol in the explicit acknowledgement request frame is larger than avalue of a sequence number of a last received MAC frame, the formingdevice forms an acknowledgement frame containing an acknowledgementbitmap which indicates that no MAC frame is successfully received, inresponse to the acknowledgement request frame.
 10. The apparatusaccording to claim 8, wherein on the basis of comparison of a value ofacknowledgement start point sequence control in the retransmittedacknowledgement request frame with a value of a sequence number of alast received MAC frame, and in response to the acknowledgement requestframe, the forming device sets a sequence number of a first successfullyreceived MAC frame as the value of acknowledgement start point sequencecontrol, and transmits an acknowledgement frame having the sequencenumber of the first MAC frame as a start point.
 11. A wirelesscommunication method comprising: forming one physical frame in which aplurality of MAC frames are aggregated; transmitting the physical frame;and receiving an acknowledgement frame which is a frame responding to animplicit acknowledgement request of the physical frame.
 12. A wirelesscommunication method comprising: receiving one physical frame in which aplurality of MAC frames are aggregated; forming an acknowledgement framewhich is a frame responding to an implicit acknowledgement request ofthe received physical frame, and contains reception statuses of theplurality of MAC frames in the physical frame as an acknowledgementbitmap, and in which a sequence number of a first successfully receivedMAC frame in the physical frame is set as an acknowledgement start pointsequence number; and transmitting the acknowledgement frame.